The invention comprises a process for forming high precision apertures in plastic components. In the presently preferred embodiment, the invention is applied to the fabrication of the tang and bore holes of a magnetic transducer positioning carriage for a magnetic disk drive assembly.
Magnetic transducer supporting carriages travel along polished guide rods which slide through the tang and bore holes of the carriage. In order to insure accurate head positioning, the tang and bore hole dimensions must be very precise. Sideplay between the carriage and the rods must be held to very tight tolerances. The bore holes must also be precisely aligned to avoid undue friction. Too much sideplay, or misalignment between the bore holes, not only causes errors in head positioning but also contributes to audible noise which is very unacceptable. Imprecise fabrication of the tang also produces excessive friction and noise.
In the past, two methods have been used to form the tang and bore holes of a carriage: molding and machining. Molding has proved to be incapable of providing the tight tolerances required due to shrinkage and wharpage. Machining has also proved to be unacceptable in that fiber reinforced compositions are used for the carriages such as polycarbonate reinforced with glass or carbons fibers. Attempts at boring carriages of this composition have yielded only very rough holes since the tool cannot cut smoothly as it encounters very wide variations in stiffness as it passes alternatively through the polymer and the fibers.